I've had a stock Folgertech FT5 R1 for about a year now and have hundreds of hours of good successful prints with it. Many have modified theirs and that's part of the fun of printing, modifying and improving the machine. Of course many mods have perceived improvements of print quality, many are just wish-list, and some are actual worthwhile improvements. The only thing that has bugged me about the physical part of the machine is the single linear rail on the X axis, the rail that supports the extruder. Linear bearings are very good in limiting "play" in many directions, but due to the nature of the bearings there is inherent play in a "rocking" moment - in the case of the FT5, this translates into play fore and aft at the nozzle end. My answer to solve this and remove all unsupported movement is to add an additional linear bearing and rail assembly under the existing gantry extrusion. The rail is readily available from Amazon in a 350mm length and the cost is low. Link to RailThe attached files and pictures show how I accomplished this. Essentially an elongated version of the melamine FT5 carriage, with some improvements in rigidity and print cooling. Of course we lose 15mm or so in Z capability, a small price. If you only have one printer you'll have to print this first of course. The assembly procedure is to leave the existing rail in place, mount the new carriage to the top, then tighten, shim, adjust the bottom rail bearing until free movement is achieved. Patience will be rewarded with rock solid, smooth moving carriage that will be free of play and increased resolution tightness layer by layer.The carriage is available on Thingiverse - Link to Carriage

I wonder if the aluminum extrusion is even necessary once you have the extra strength of a second rail? Maybe the whole assembly could be simplified to just a pair of the linear rails, directly bolted to each other, as the X-axis span?

I suspect that the problem would then become the attachment points of the longer (top) slide to the two Y carriage rails.They might be too flexible.Also, part of the appeal of a mod such as this is that the two slides are separated by some distance, in this case of course- 20mm.That makes a wider base distance upon which torque forces from extruder carriage inertia would be acting.

I've been experiencing play at the nozzle because of the play in the X bearing too and saw this and it looks pretty neat but not sure about the weight it'd add to the assembly. But I wonder if a similar effect could be attained by having a v-rail wheel ride the bottom of the aluminum extrusion.

Hey, Ben - Your thoughts about a v-rail bearing are also good ones. Many 3D printers use just that for most if not all of their guides, the CR10 comes to mind - and they get really good results too. You'll have to redesign the mount to accommodate the wheel and axle and more importantly, the adjustment that will be needed to get the right amount of resistance. - Good luck and let us know how it turns out.

Also, to answer your question about weight - the real issue to consider is inertia and the speed at which you print - I very rarely print over 40mm/sec, but I feel this setup would be just fine for up to 60mm/sec. I've had no issues with the modification.

1, Design a sling that goes under the motor and back onto the front of the backboard.2, Copy the design with Fusion and send the file to be cut in thick Carbon fiber, no flex in that stuff!3, Wait around till the alloy upgrade is available

The bearing it'self is next to zero, WAAAAY better than the poly wheels that the CR10 uses (axial bearings have issues with side load an play, when I had the CR10 I was driven batty with the bearing play)

The problem is the ACM, its stiff, but not stiff enough IMHO for direct extruder cradle duty.